1. Field of the Invention
This invention relates generally to Josephson transmission lines, and more particularly, to Josephson transmission lines that employ underdamped Josephson junctions to enhance the speed of signal propagation.
2. Discussion of the Related Art
With recent developments in superconductor technology, superconductor devices based on Josephson Junctions effect are replacing conventional devices based on semiconductor technology for high performance and low power. Digital circuits that employ superconductor technology are often desirable because these devices can simultaneously consume very little power and operate at very high clock speeds as compared to their semiconductor counterparts. Because of low power consumption, it is possible to make systems very compact. Other benefits for signal transmission using superconducting devices include reduced signal attenuation and noise. Digital circuits that employ superconductor devices can operate at clock speeds exceeding 100 GHz.
Josephson transmission lines (JTL) are typically employed in superconductor digital circuits to manipulate and transmit narrow pulse width signals at low power. JTLs employ Josephson junctions at predetermined intervals along the transmission line that regenerate and transmit pulse signals as a single flux quanta (SFQ), or a single quanta of magnetic flux. The Josephson junction functions as a tunneling device that includes two opposing superconductive films, separated by an oxide dielectric layer. A current bias is applied to each Josephson junction. These junctions then switch or flip in response to an incoming transient voltage pulse, regenerating that pulse for the next junction, and returning to their initial state where they are ready to respond to the next pulse. Each Josephson junction generates a voltage pulse when it switches. Typical SFQ pulse signals generated by a Josephson junction are 2-3 ps in width and 1 mV. The time integral of the voltage pulse is equal to a single quanta of magnetic flux "PHgr"0=2.07xc3x9710xe2x88x9215 Volt-seconds.
FIG. 1 is a schematic diagram of a standard superconducting Josephson transmission line (JTL) 10 that is representative of the known transmission lines of this type. The JTL 10 is comprised of a sequence of JTL segments 11. The JTL segment 11 includes an inductor 16 connecting adjacent junctions, a resistively shunted junction circuit 18 connecting one end of inductor 16 to a common ground return, and a biasing resistor connected between the top of the junction 18 and a current source 12. The isolation inductor 16, provides inductive isolation between adjacent junctions 11 and allow propagation of the SFQ pulse along the JTL 10. The biasing resistor 14 is connected in series with a current source 12 which provides an equal amount of current to each of the Josephson junction circuits 18 and 20.
The Josephson junction circuits 18 and 20 of the JTL 10 are spaced apart at predetermined intervals along the JTL 10 and act to regenerate the SFQ pulses at each stage. Each Josephson junction circuit 18 and 20 is shown as an equivalent circuit of a resistor and Josephson junction in a parallel array. The equivalent elements of the JTL segment 11 and the Josephson junction circuit 18 will be described with the understanding that all of the Josephson junction circuits in the JTL 10 have the same elements. The Josephson junction circuit 18 includes a Josephson junction 22 that is connected in series with a first parasitic inductor 24. The Josephson junction 22 and the first parasitic inductor 24 are connected in parallel with a damping resistor 26 and a second parasitic inductor 28. The first and second parasitic inductors 24 and 28 are connected to a reference ground 30 opposite the Josephson junction 22 and the damping resistor 26. The damping resistor 26 shunts the Josephson junction 22 and helps to define its response to incoming signals. The damping resistor 26 is chosen such that the Stewart-McCumber parameter [W. C. Stewart, Applied Physics Letters 12, 277 (1968). D. E. McCumber, Journal of Applied Physics 39, 3113 (1968)], which parameterizes how a Josephson junction is damped, falls between 1 and 2.
When an SFQ pulse impinges the JTL segment 11, the Josephson junction 22 flips, or increments its internal degree of freedom, or phase by 2xcfx80. When the Josephson junction 22 flips, the Josephson junction 22 regenerates and transmits an SFQ pulse to the next junction. When the next junction receives the SFQ pulse, it recreates and propagates the SFQ pulse to the following junction. Typically, the travel time of the SFQ pulse from one Josephson junction to the other ranges between 2.5-4 picoseconds(ps) depending on the degree of damping (Stewart-McCumber parameter) and the current bias. At any given time, at least two junctions are in the process of advancing their phase.
When the junction flips, the Josephson junction 22 regenerates a voltage pulse having a fixed time integral "PHgr"0. In cases where the junction carries current that is less than a predetermined threshold, the Josephson junction does not flip in response to the input pulse and fails to regenerate and retransmit the voltage pulse to the next junction. On the other hand, when the junction carries current that exceeds a predetermined threshold, the Josephson junction goes into a voltage state where it emits a pulse train, or multiple voltage pulses in rapid succession although only one pulse is expected, and leads to erroneous results of a circuit. Damping resistor 26 helps to prevent JTL 10 from going into the voltage state. Conventionally the magnitude of the inductance of the isolation inductor 16 is such that the product of its inductance and the Josephson junction""s critical current (the L-Ic product) is in the range 0.7-1.0 milliAmp-picoHenrys. In addition, each junction 11 is current biased with about 60%-80% of the critical current of the Josephson junction.
Furthermore, as the SFQ pulse is transmitted down the JTL 10, the damping resistor 26 generates Johnson noise, or current noise that effects the junction 22. Because the speed of propagation depends upon the applied current bias, the current noise results in timing jitter, or uncertainty in the time of flight of the SFQ pulse. Additionally, because the Johnson noise applied to each junction 22 is independent with respect to the other junction 11, the timing jitter of each junction 22 is also independent with respect to the other junction. The accumulation of this timing jitter in the JTL 10 increases in proportion to the square root of the number of JTL segments 11 in JTL 10. The damping provided by the damping resistor 26 and the first and second parasitic inductors 24 and 28 increases the propagation delay and results in a slower speed of signal propagation on a chip as could otherwise be realized. The propagation speed of an SFQ pulse along JTL 10 is about one tenth of the propagation speed along a passive microstrip transmission line fabricated in the same superconducting integrated circuit (IC) technology. Therefore, the standard JTL 10 available for SFQ based devices is slow, and adds timing jitter or timing uncertainty to the signal it carries. This timing uncertainty ultimately limits the maximum clock speed of the clocked logic circuits and the performance of superconducting devices that employ Josephson junction transmission lines.
What is needed is a superconductor Josephson transmission line that provides the transmission and distribution of the SFQ pulses on a superconducting integrated circuit without suffering from the drawbacks discussed above. It is therefore an object of the present invention to provide a superconducting Josephson transmission line that transmits the SFQ pulses at faster speeds with reduced timing jitter.
In accordance with the teachings of the present invention, a Josephson transmission line (JTL) for transmitting single flux quantum pulses is provided. The JTL includes a current source, a plurality of isolation inductors electrically coupled in series along the JTL, and a plurality of Josephson junction circuits electrically coupled in parallel along the JTL. Each of the Josephson junction circuits includes a Josephson junction and a parasitic inductor coupled in series. However, the Josephson junction circuits do not include any damping elements. This enables the JTL to reduce timing uncertainty and to enhance propagation speed. In one embodiment, each Josephson junction circuit is biased with about 30% of the critical current of the Josephson junction and the inductance of the isolation inductor is adjusted between 0.2-0.4 pHmA/IC, where IC is the critical current in mA of the Josephson junction. The reduced current bias and inductance of the isolation inductor help prevent the transmission line from going into a voltage state in the absence of damping resistors. The present invention, thus, provides a superconducting Josephson transmission line that transmits and distributes the SFQ pulses at faster speeds with reduced timing jitter.
Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims.